The motor specifications themselves aren't very useful when it comes to questions like this. Inductance, maximum torque - yes, look at the spec. But forget the nominal voltage ratings (within reason - but a typical 68V supply is about right for NEMA23). Similarly, the Gecko "rules" are very rough starting points and are not hard-and-fast rules. Don't forget that the stepper driver sits between the power supply and the motor. The driver will limit current and as long as you don't exceed the driver voltage ratings, the more volts the better the motor torque at speed (as mentioned in previous post). Best advice is generally to start with the driver current setting set to the nominal motor current as per spec sheet, then check the motor temperature after it has been running for a little while. If it's a bit too hot to hold for any length of time, drop the current setting by one step. If it's much cooler than that, increase the motor current setting. That equates to a motor surface temperature of about 60degC at which point the internal motor temp shouldn't be too high. A crude but effective way to set motor current! My own motors have been set and run on that basis for a few years now with absolutely no problems.